(grep-program): New variable.

(grep-program): New variable.
(grep-command): Use it, and test whether it supports the -e
option; fix doc string (last command is stored in history variable).
(grep-null-device): Declare before grep-program and grep-command.
(grep-find-use-xargs, grep-find-command, grep-find-history):
New variables.
(grep-find): New command.
(grep): Only concatenate grep-null-device to COMMAND when it's
not nil (to support grep-find).